1. Field of the Invention
The present invention relates to a magnetic resonance image diagnostic apparatus fit for providing susceptibility-weighted images of the interior of, for example, the head, and also to a method of controlling the apparatus.
2. Description of the Related Art
T2*-weighted images acquired by the gradient echo method acutely reflect the non-uniformity of a local magnetic field. Therefore, the gradient echo method is widely used as an imaging method for visualizing a difference between regions of interests in terms of susceptibility. In order to provide T2*-weighted images of the subject's head, rephasing is generally performed by means of gradient moment nulling (GMN), nullifying the influence that the blood flow imposes on the image quality. T2*-weighted images are thereby acquired. The longer the echo time, the more acutely any T2*-weighted image will reflect the difference in susceptibility. This is why the echo time is set relatively long in most cases to provide T2*-weighted images. An imaging method that is more sensitive to changes in susceptibility than T2*-weighted imaging is proposed in Magn Reson, Med. 52:612-618, 2004 (hereinafter referred to as the reference document). In this imaging method, it is possible to enhance both effects of amplitude reduction and phase difference induced by susceptibility effects because a phase-shift weighted mask is multiplied by amplitude images.
FIG. 14 is a diagram showing the pulse sequence in the three-dimensional (3D) gradient echo method that performs a first-order GMN in three axes, i.e., slice, phase encode and read-out. Assume that a phase shift occurs, due to a difference in susceptibility only, at the time an echo peak is observed. Then, the shift is proportional to the difference in susceptibility. If the phase change due to susceptibility is not taken into consideration, the phase shift of the spin, which results from the application of a gradient magnetic field, is given by the following equation (1) by using the period corresponding to the echo time TE as integration section.
                    -                  γ          ⁡                      (                                          r                ⁢                                                                  ⁢                0                ⁢                                  ∫                                                            G                      ⁡                                              (                        t                        )                                                              ⁢                                          ⅆ                      t                                                                                  +                              v                ⁢                                                                  ⁢                0                ⁢                                  ∫                                                                                    G                        ⁡                                                  (                          t                          )                                                                    ·                      t                                        ⁢                                          ⅆ                      t                                                                                  +                                                                    a                    ⁢                                                                                  ⁢                    0                                    2                                ⁢                                  ∫                                                                                    G                        ⁡                                                  (                          t                          )                                                                    ·                                              t                        2                                                              ⁢                                          ⅆ                      t                                                                                  +              …                        )                                              (        1        )            
where γ is a gyromagnetic ratio of about 2π×42.6 MHz/T. G(t) is the gradient waveform vector, which corresponds to Gss, Gpe and Gro for the three axes (i.e., slice, phase encode and read-out), respectively, and r0, v0 and a0 are position, velocity and acceleration vectors of the spin at time t (t=0), respectively.
In equation (1), the respective terms indicate phase changes according to the position, velocity and acceleration, and correspond to the 0th-order, 1st-order and 2nd-order gradient moments. In equation (1), the third-order moment or any higher moment are not described. Nonetheless, the third-order moment and any higher-order moment contribute to the phase change, too.
GMN is a process that determines G(t) so that any gradient moment up to a certain order may have the minimum value such as 0 for the echo time TE. GMN is therefore also called rephasing, too. In the phase encode or slice encode, however, the 0th moment changes every time an encode step is performed. Thus, in the GMN for the phase-encode axis, the 0th moment at the echo time TE is set to a specific value assigned to each encode step, and the first-order moment and any higher-order moment are set to a value as small as possible, such as 0. The zeroth-order GMN alone cannot achieve rephasing on the first-order moment and any higher-order moment in equation (1) for, for example, the blood flow that has active spins. Inevitably, the phases change as the object moves. Consequently, phase dispersion takes place, reducing the vector sum of the blood-flow spins, and the blood flow is therefore represented by a lower-level signal. Since the phase disperses in various ways in accordance with the velocity at which the blood flows, some blood flows may undergo insufficient phase dispersion and may not be represented by signals of sufficiently low level. In some cases, the blood flows will result in artifacts. Hence, in order to provide a T2*-weighted image of, for example, the head, which is free of influence of any fluid flow such as blood flow, the first-order GMN or any higher-order GMN must be performed. How high the order of GMN should be depends on whether G(t) can be rephased within the echo time TE at that order. The higher the order of the rephasing, the more greatly the influence of the flow can be reduced.
The reference document proposes that a T2*-weighted image of the head, obtained by the gradient echo method of performing GMN of the first order in the three axes, be phase-weighted so that the image may be further susceptibility-weighted. This proposed method will be explained below.
The original amplitude image, which will be phase-weighted, is an image that reflects a difference in susceptibility because it is free of the influence of the blood flow, owing to the first-order GMN. In the phase data, the second-order moment and any higher-order moment is not rephased in equation (1). Nonetheless, the susceptibility is considered to dominantly control the phase shift of the spin since the phase shift is almost free of the influence of a flow such as the blood flow. The phase shift can therefore be regarded as representing the difference in susceptibility between different kinds of tissue. Thus, a phase mask image that lowers the signal value in proportion to the phase shift may be generated from the phase data and then repeatedly applied to the amplitude image several times. Then, an image further phase-shift-weighted in accordance with the susceptibility can be acquired. The image acquired by this method reflects the difference in susceptibility between different kinds of tissue in the form of a difference in contrast. In the head, the difference between the venous blood having a high deoxyhemoglobin concentration and the surrounding tissue having a high oxyhemoglobin concentration is large in terms of susceptibility. An image clearly visualizing the veins can therefore be provided if phase weighting is applied to the T2*-weighted image of the head. This method has been reported to be useful and effective if used in BOLD venography.
The technique disclosed in the reference document can indeed provide susceptibility-weighted images that visualize the veins more clearly than is possible before. However, these images do not visualize the arteries more clearly, because the arteries exhibit less non-uniformity of susceptibility than the veins. Inevitably, the effect of any flow cannot contribute to image contrast. With the technique disclosed in the reference document, the phase masking must be repeated a plurality of times in order to enhance the vein-visualizing ability as much as desired. If the phase masking is repeated, however, the phase difference between the veins and the surrounding tissue will be emphasized. Consequently, the vascular cavities of veins may be over-evaluated or the artifacts may be emphasized due to the susceptibility effect.
The technique disclosed in the reference document can certainly generate phase masks reflecting phase changes that dominate susceptibility only. However, second-order GMN or any higher-order GMN is not performed, and only first-order GMN is performed. Hence, the technique cannot prevent artifacts that develop from second-order moments and higher-order moments generated as the blood vessels undergo a pulsating motion or a complex motion.